1. Field of the Invention
The invention relates to digital video decoders and in particular to multi-directional comb filtering in a digital video decoder.
2. Background of the Invention
Due to advancing semiconductor processing technology, integrated circuits (ICs) have greatly increased in functionality and complexity. With increasing processing and memory capabilities, many formerly analog tasks are being performed digitally. For example, images, audio and full motion video can now be produced, distributed, and used in digital formats.
Although digital images generally provide higher noise immunity, most digital images in digital video streams are converted from analog video streams. The original analog video stream may contain noise from various sources. For example, modulation, wireless transmission and demodulation of TV signals may introduce Gaussian-like noise. Furthermore, even analog video transferred over transmission lines may have Gaussian-like noise due to magnetic fields around the transmission lines. In addition, the digitalizing process may inadvertently amplify minor noise problems in the analog video stream. For more information on methods of noise reduction for interlaced digital video stream, see United States Patent Publication 20070103594 (application Ser. No. 11/644,855 by Zhu, published on May 10, 2007, filed on Dec. 22, 2006, titled “Recursive noise reduction with still pixel detection” and assigned to Huaya Microelectronics, Ltd.), the contents of which are incorporated herein by reference.
FIG. 1A is an illustrative diagram of a portion of interlaced digital video signal 100′ most often used in television systems. Interlaced digital video signal 100′ comprises a series of individual fields F(0) to F(N). Even fields contain even numbered rows while odd fields contain odd numbered rows. For example if a frame has 400 rows of 640 pixels, the even field would contains rows 2, 4, . . . 400 and the odd field would contains rows 1, 3, 5, . . . 399 of the frame. In general, for an interlaced video signal each field is formed at a different time. For example, an interlaced video capture device (e.g. a video camera) captures and stores the odd scan lines of a scene at time T as field F(5), then the video capture device stores the even scan lines of a scene at time T+1 as field F(6). The process continues for each field. Two main interlaced video standards are used. The PAL (Phase Alternating Line) standard, which is used in Europe, displays 50 fields per seconds (fps) and the NTSC (National Television System Committee) standard, which is used in the United States, displays 60 fps. Interlaced video systems were designed when bandwidth limitations precluded progressive (i.e., non-interlaced) video systems with adequate frame rates. Specifically, interlacing two 25 fps fields achieved an effective 50 frame per second frame rate because the phosphors used in television sets would remain “lit” while the second field is drawn.
To ease transmission of video signals, chrominance information and luminance information are combined via modulation into a single composite video signal. Imperfect decoding of composite video signals in either PAL or NTSC format may lead to color-crossing. Specifically, color-crossing error often appears in a video image where the local luminance spatial frequency is near the sub-carrier frequency of the chrominance information. Color-crossing errors occur in both PAL and NTSC video signals.
For example, NTSC video signals typically have a chrominance sub-carrier frequency of 3.58 MHz, i.e., chrominance information is modulated by a sinusoid signal with a frequency equal to 3.58 MHz before transmission. Luminance information may also have components that overlap with the chrominance information near the chrominance sub-carrier frequency. Thus, the luminance components near the chrominance sub-carrier frequency cause color-crossing errors, which cannot be cleanly removed. Generally, during video decoding a band pass filter at the chrominance sub-carrier frequency is used to obtain the chrominance information. However, the luminance components, which are near the chrominance sub-carrier frequency, are not blocked by the band pass filter. Therefore, the decoded chrominance signal would include “unclean” chrominance information. The color-crossing errors produce rainbow like color blinking in the decoded video image. In PAL video signals, the same color-crossing errors also occur at the PAL chrominance sub-carrier frequency of 4.43 MHz. Color-crossing error can also occur in other encoded video signals.
Conventionally, 3D comb filters have been used to reduce color-crossing errors. Specifically, in NTSC composite video signals the chrominance of corresponding pixels in two consecutive fields of the same type (odd or even) have a phase difference equal to 180 degrees. A 3D comb filter can cancel the miss-included luminance components by a simple subtraction of the video signal values of the two corresponding pixels, when the video image is not changing. However, for PAL composite video, the chrominance of corresponding pixels in two consecutive fields of the same type have only a 90-degree phase difference. Thus, to use 3D comb filters to correct color-crossing errors in decoded PAL composite video signals, four fields must be used.
While 3D comb filters can reduce color-crossing errors, 3D comb filters may also degrade other aspects of video quality. For example, 3D comb filters are very sensitive to noise in composite video signals; therefore, a digital video decoder with a 3D comb filter would have difficulties with weak video signals, which are common in many areas. Furthermore, high quality 3D comb filters are very expensive relative to other components of a video system. For more information on efficient reduction of color-crossing errors from decoded composite video signals, see United States Patent Publication 20060092332 (application Ser. No. 11/046,591 by Zhu, published on May 4, 2006, filed on Jan. 28, 2005, titled “Color-crossing error suppression system and method for decoded composite video signals” and assigned to Huaya Microelectronics, Ltd.), the contents of which are incorporated herein by reference.
Modern video signals typically consist of a sequence of still images, or frames or fields as described above. By displaying the sequence of images in rapid succession on a display unit such as a computer monitor or television, an illusion of full motion video can be produced. A standard NTSC television display has a frame rate of 29.970 fps (frames per second). For historical reasons, the frames in video displays for most consumer applications (and many professional applications) are formed from “interlaced” video signals in which the video signals are made up of “fields” that include half the data required for a full frame. As described above, each field includes every other row of pixels that would be included in a complete frame, with one field (the “odd field”) including all the odd rows of the frame, and the other field (the “even field”) including all of the even rows.
FIG. 1B depicts this interlacing concept, as a view 110′ is interlaced into an odd field 120′ and an even field 130′. Odd field 120′ includes odd rows SO(1), SO(2), SO(3), SO(4), SO(5), SO(6), SO(7), and SO(8), which represent rows 1, 3, 5, 7, 9, 11, 13, and 15, respectively, of view 110′. Even field 130′ includes even rows SE(1), SE(2), SE(3), SE(4), SE(5), SE(6), SE(7), SE(8), which represent rows 2, 4, 6, 8, 10, 12, 14, and 16, respectively, of view 110′. Note that each of odd rows SO(1) SO(8) in field 120′ corresponds to a blank row (i.e., a row with no pixel values) in field 130′, while each of even rows SE(1) SE(8) in field 130′ corresponds to a blank row in field 120′.
View 110′ depicts a white square 111′ formed in a shaded background 112′. Therefore, odd rows SO(1) SO(8) are all shaded, except for a white portion 121′ in each of odd rows SO(4), SO(5), and SO(6) corresponding to the portion of those rows corresponding to white square 111′. Similarly, even rows SE(1) SE(8) are all shaded, except for a white portion 131′ in each of even rows SE(3), SE(4), and SE(5), corresponding to the portion of those rows corresponding to white square 111′.
Note that color video signals contain chrominance and luminance information. Chrominance is that portion of video that corresponds to color values and includes information about hue and saturation. Color video signals may be expressed in terms of RGB components: a red component R, a green component G, and a blue component B. Luminance is that portion of video corresponding to brightness value. In a black and white video signal, luminance is the grayscale brightness value of the black and white signal. In a color video signal, luminance can be converted into red, green and blue components, or can be approximated by a weighted average of the red, green and blue components. For example, in one well-known scheme, luminance is approximated by the equation: Y=0.30*R+0.59*G+0.11*B. For explanatory purposes, shaded regions of the figures represent lower luminance values than blank (white) regions. For example, the white portion 121′ in odd row SO(4) has a higher luminance value than the shaded portion of the same row.
To generate a progressive (i.e., non-interlaced) video display from an interlaced video signal, the video signal must be de-interlaced. Conventional de-interlace methodologies can be divided into two main categories: (1) 2D de-interlacing; and (2) 3D de-interlacing. In 2D de-interlacing, a frame is recreated from a single field via interpolation of the rows in that field. A common 2D de-interlacing technique involves duplicating each row of a single frame to provide pixel values for the blank rows; i.e., each blank row in an odd field could be filled with a copy of the odd row directly below that empty row, while each blank row in an even field could be filled with a copy of the even row directly above that empty row. The 2D de-interlacing is particularly useful for scenes involving fast motion since even if a scene change occurs between consecutive fields, such changes would not distort a frame formed using “pure” common-field pixel interpolation (i.e., formed using only the pixels in a single field). For additional information on 2D and 3D mixing, see U.S. Pat. No. 7,142,223 (application Ser. No. 10/659,772 by Zhu, issued on Nov. 28, 2006, titled “Mixed 2D and 3D de-interlacer” and assigned to Huaya Microelectronics, Ltd.), the contents of which are incorporated herein by reference.
Hence, there is a need for improved digital video decoders to minimize distortion in a recreated image.